This invention relates to digital computer network technology. More specifically, it relates to methods and apparatus for synchronizing components within the Head End of an access network.
In conventional Data Over Cable Service Interface Specification (DOCSIS) systems, there may be multiple Cable Modem Termination Systems (CMTSs) each including a plurality of physically distinct line cards having appropriate hardware for communicating with cable modems in the network. Each CMTS and each line card is typically assigned to a separate DOCSIS domain, which is a collection of downstream and upstream channels.
Typically, each DOCSIS domain includes a single downstream channel and one or more upstream channels. The downstream channel is used by the CMTS to broadcast data to all cable modems (CMs) within that particular domain. Only the CMTS transmits data on the downstream. In order to allow the cable modems of a particular DOCSIS domain to transmit data to the CMTS, the cable modems share one or more upstream channels within that domain.
Access to the upstream channel is controlled using a time division multiplexing (TDM) approach. Such an implementation requires that the CMTS and all cable modems sharing an upstream channel within a particular domain have a common concept of time so that when the CMTS tells a particular cable modem to transmit data at time T, the cable modem understands what to do. “Time” in this context is tracked using a counter, commonly referred to as a timestamp counter, which, according to conventional implementations is a 32-bit counter that increments by one every clock pulse.
In conventional configurations, each line card in each CMTS may have its own unique timestamp counter which generates its own local time reference. Thus, each line card in the DOCSIS system operates according to its own local time reference, and is not synchronized with other line cards in the same CMTS or with line cards in other CMTSs.
Each line card in the DOCSIS system periodically distributes a timestamp value of its local time reference to the respective group of cable modems serviced by that line card. For this reason, a first group of cable modems serviced by a first line card in a first CMTS will not be synchronized with a second group of cable modems serviced by a second line card in a second CMTS.
The present invention addresses this and other problems associated with the prior art.